The mutual tooth engagement of gearwheels fitted without clearance occurs at their respective reference or pitch circles. The between-centers distance (the center distance) between two gearwheels fitted without clearance will then be the sum of the radii of their respective reference or pitch circles. To cater for thermal expansion, the center distance between the gearwheels is usually increased somewhat to create a flank clearance between them. Otherwise, such expansion would cause large radial forces between them. Excessive radial forces between the gearwheels would increase the load on each gearwheel bearing, shortening its service life. Large radial forces would also increase the wear on the flanks of the gearwheels' teeth, potentially leading to damage to the teeth. The amount of flank clearance also affects the transmission ratio between the gearwheels. To achieve a constant ratio, the contact perpendicular of a gear pair in engagement should always run through a fixed point on a straight line between their axes. The noise generated by the gearwheel transmission will also be affected by the flank clearance.
Gearwheel transmissions are used in different contexts in a large number of different mechanical configurations. The flank clearance between the cooperating gearwheels in such configurations is of great significance for the achievement of a functioning and reliable configuration.
A gearwheel transmission used to drive the camshaft of a combustion engine has to fulfil a number of factors. To achieve precise opening and closing times for the engine's valves, the flank clearance between the cooperating gearwheels must be adjusted accurately. At the same time, as quiet a transmission as possible is desired with a view to preventing noise which might adversely affect the surroundings. If the engine is intended to power a vehicle, there are in certain countries legal requirements about how high a noise level the engine is allowed to generate.
The gearwheel transmission used to drive the camshaft often comprises a number of gearwheels in mutual engagement. If the engine has one or more overhead camshafts, the distance between its crankshaft and the camshafts will be considerable. The transmission between them has therefore to comprise a number of gearwheels. The crankshaft is fitted in the engine block and the overhead camshafts are fitted in the cylinder head. The manufacturing tolerances of engine blocks and cylinder heads and the respective seals and gaskets between these components in the fitted state are such that the distance between crankshaft and camshafts will vary between engines of the same type and make. This means that the center distances of the gearwheels will vary because the engine's components are manufactured with a certain tolerance.
Compression of the cylinder head gasket may for example occur during normal operation of the engine. The engine's servicing and repair may entail replacement or reconditioning, e.g. by grinding, of the cylinder head. The center distances of the gearwheels may therefore change both during operation and as a result of servicing and repair of the engine.
A known practice is to provide gearwheel systems with clearance adjustment mechanisms so that the flank clearance settings between the cooperating gearwheels can be altered to cater for the way their flank clearance is affected by normal operation, wear, servicing and repairs.
Another known practice is to employ clearance adjustment methods whereby an intermediate pinion is provided on an adjustable bracket for adjusting the flank clearance between the cooperating gearwheels.
WO9300530 refers to a mechanism for adjusting the center distance between gearwheels in a gearwheel transmission between a combustion engine's crankshaft and camshaft. An intermediate gearwheel can be turned about an axis of rotation when screws fixing it are slackened. The gearwheel has oblong recesses in which the fixing screws can travel. When the center distance is correctly adjusted, the fixing screws are tightened so that the position of the gearwheel is fixed relative to the engine block.
U.S. Pat. No. 3,502,059 refers to an adjusting mechanism for gearwheels in a gearwheel transmission between a combustion engine's crankshaft and camshaft. An intermediate gearwheel is provided on a pivotable bracket. When a number of bolts fastening it are released, the bracket can be turned about a pivot pin. When the intermediate gearwheel is correctly adjusted, the fastening bolts are tightened so that the positions of the bracket and the intermediate gearwheel are fixed relative to the engine's cylinder head.